1. Field of the Invention
The present invention relates generally to gas sensors with attenuated drift characteristics, and in a specific aspect to a hydrogen gas sensor including a palladium/yttrium layer structure, such as may be employed in environments susceptible to generation or incursion of hydrogen gas. More generally, the invention relates to a sensor with a sensing layer whose response to a target species is subject to drift in extended use.
2. Description of the Related Art
U.S. Pat. No. 6,006,582 issued Dec. 28, 1999 in the names of Gautam Bhandari, et al. for “Hydrogen Sensor Utilizing Rare Earth Metal Thin Film Detection Element,” discloses a hydrogen sensor including a rare earth metal thin film arranged for exposure to the environment and exhibiting a detectable change of physical property, e.g., electrical conductance, electrical resistance, electrical capacitance, magnetoresistance, etc., when the rare earth metal thin film is contacted with hydrogen gas. The sensor is operatively coupled with an output assembly for converting the physical property change to a perceivable output in response to the presence of hydrogen in the monitored gas environment.
U.S. Pat. No. 6,265,222 issued Jul. 24, 2001 in the names of Frank DiMeo, Jr., et al. for “Micro-Machined Thin Film Hydrogen Gas Sensor, and Method of Making and Using the Same,” discloses a hydrogen sensor including a thin film sensor element of similar type formed, e.g., by metalorganic chemical vapor deposition (MOCVD) or physical vapor deposition (PVD) of a hydrogen-interactive metal film on a micro-hotplate structure. The hydrogen-interactive metal film in a preferred embodiment is overcoated with a thin film hydrogen-permeable barrier layer to protect the hydrogen-interactive film from deleterious interaction with non-hydrogen species.
When micro-hotplate hydrogen gas sensors of the foregoing type are utilized with yttrium as the hydrogen-interactive metal film and palladium as the thin film hydrogen-permeable barrier layer to protect the yttrium film from interaction with non-hydrogen species, the performance of the sensor initially is highly effective, but degrades with time.
This progressive degradation is manifested as drift in the output signal derived from the sensor, and is of sufficient magnitude to seriously adversely impact the accuracy and service lifetime of the sensor. Drift rates well in excess of 5% per day have been documented for micro-hotplate hydrogen sensors having a Pd/Y bilayer structure formed on a silicon dioxide insulating layer mounted on aluminum contact pads. Such rates of degradation severely compromise the usefulness of the hydrogen sensor, requiring constant recalibration if the sensor is to used for extended in-service operation, and raising the possibility that significantly increased concentrations of hydrogen may be present in the monitored gas environment before detection occurs, relative to detection capability of the sensor at inception of use.
It would therefore be a significant advance in the art to provide a reliable hydrogen gas sensor structure that avoids the foregoing problems, and is characterized by a very low rate of drift in use.